The use of radiation in the range from approximately 10 gigahertz (GHz) to approximately 100 GHz in non-destructive material testing and for full-body scanners is already known. The corresponding millimeter waves or respectively microwaves are suitable, for example, for the detection of metallic, but especially also non-metallic objects, for example, objects made from ceramic materials or certain synthetic materials. At the same time, air and materials such as those used, for example, for clothing, are transparent for such radiation. Accordingly, millimeter waves or microwaves are suitable, for example, for contactless detection of objects concealed beneath the clothing. Corresponding scanners can supplement or replace metal detectors, for example, in the field of security checks at airports or security-relevant public facilities.
During the operation of such a scanner, a microwave signal with a given bandwidth is generated and transmitted from at least one antenna in the direction towards a human body, optionally with objects concealed beneath the clothing. The microwave signals are reflected from the body and from the concealed objects. The reflections are received by at least one antenna. Separate transmitting and receiving antennas can be provided.
A scanner can also include a plurality of transmitting and/or receiving antennas. Such a scanner can be operated, for example, in a multi-static mode, in which a transmitting antenna transmits a signal, and reflections of this signal are received according to amplitude and phase position by several receiving antennas or by all receiving antennas, after which the next transmitting antenna transmits a signal etc.
Methods for aperture synthesis can also be used, for example, methods of digital beam forming (“Digital Beamforming”, DBF), in which a focusing of the transmitted and/or received radiation can take place for the scanning of objects without mechanical movement of the antennas or electromagnetic bundling through lenses or similar. Instead, a focusing is implemented only through targeted control of the transmitting antennas and/or evaluation of the signals detected by the receiving antennas. In the latter case, a signal reflected from a given spatial point by a software-based algorithm in the direction towards several antennas and received there, is evaluated accordingly.
Even in the case of optimised hardware and with the use of highly developed signal processing, there is a continuing general requirement for an improvement in the detection capability of such scanners. It should be possible to obtain views or images of people and objects with sufficiently clear contours, so that an automatic detection, for example, of undesirable objects, is possible, and/or so that security personnel can be supported with the most reliable detection possible. In this context, the measurement times should be as short as possible, so that people can be scanned, for example, in an undisturbed movement flow.
Efforts have been directed primarily towards an improvement of illumination. For example, WO 2012/167847 A1 proposes the additional provision of at least one reflector element alongside an arrangement of transmitting/receiving antennas. As a result of the reflector element, a relatively larger proportion of the micrometer-wave signals can be used for the reconstruction.
U.S. Pat. No. 6,965,340 B1 describes a security gate with four panels arranged in an angled manner. A person walks in a straight line through the gate and is scanned in this context by means of microwave radiation.